Regenerative power units save energy

Jun Kang

03/13/2013

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3. Matrix converter

A matrix converter is a direct ac-to-ac power converter that has motoring and regeneration capability. Recent technology advances in power semiconductor and CPU computing power have made commercial products possible. Basic features of the matrix converter are:

- All-in-one, where ac input is directly converted into variable frequency ac output by nine bidirectional switches. So the functions of the sinusoidal regen converter and VFD are combined. This feature enables the motors’ operation in motoring and regeneration modes without an additional VFD.

- No electrolytic capacitor: Matrix converter needs less maintenance because there is no diode rectifier and dc electrolytic capacitor in the main power flow. Generally, an electrolytic capacitor is bulky and has a shorter lifetime than other components.

- Low input current harmonics: Input current control capability makes it possible to reduce input current harmonics significantly. Input current harmonic distortion is in the range of 5% to 10% under full load condition. Neither phase-shifting transformer nor external line reactor is required to reduce harmonic currents, which are usually bulky.

- Compact size: Physical size of matrix converter is smaller than the sinusoidal PWM converter-inverter system, which has motoring and regenerative power capabilities that are the same as the matrix converter. [5-7]

4. Utility cost saving by regeneration power unit

Figure 5 demonstrates the energy costs of operating a 45 kW elevator system with and without a regenerative power unit. Average motoring power consumed per cycle is 8.8 kW while regenerative power is 4.78 kW. If a dynamic braking unit is used to produce braking torque, 11,300 kWh will be converted into heat loss. By installing a regenerative power unit, 54% of power can be saved and annual utility cost savings is $1017 assuming average electricity costs of 9 cents per kWh.

5. Regeneration power unit selection

All three regeneration power units described have a good energy cost savings capability under regenerative load conditions. The regenerative converter described in section 2 is the best low-cost solution. But if extra advantages such as low input current harmonics and unity power factor are also important, the best solution may differ. The table compares various aspects of three regeneration power units.